Generally, it can
be said that the question of compounding and dispensing an
incompatible prescription, is entirely dependent upon good judgment.
Every pharmacist
should know an explosive prescription at sight, should know those
which should not be combined under any circumstances (such as
glycerin and nitric acid), and those which can be mixed if proper
precautions are taken (like iodine and oil of turpentine). Explosive
prescriptions are a menace to the Compounder rather than to the
customer, hence, self-preservation requires a complete knowledge of
such dangerous prescriptions.
In other forms of
incompatibility the pharmacist is confronted with two important
questions:
- Is the prescription safe to dispense?
- How can it be least objectionably compounded?
There are
frequently cases of intentional incompatibilities cases where the
doctor really desires an unsightly mixture, because of its
therapeutic value. We all know that the official chalk mixture is of
value because of the insoluble calcium carbonate it contains, another
is the prescription calling for zinc sulfate and lead acetate, which
must be dispensed with precipitated lead sulfate, since the
therapeutic value or its healing property is dependent largely upon
it.
On the other hand
prescription in which potent medicines are produced in insoluble
form, like prescriptions where powerful alkaoids are precipitated,
should be viewed with suspicion by the compounder, and while it is
often an exceedingly delicate matter to approach the physician on the
subject, it is usually best to let the doctor know, either at the
time, or later of the possibility of danger. No pharmacist of act or
good sense of course will bluntly announce to the customer that he
will not compound such a prescription, but will always consult the
physician. In all cases where precipitation or possible precipitation
of potent drugs will take place the container should be provided with
a “shake well” label.
There are some
prescriptions of this kind, however, which must not be dispensed,
such as the mixing of chloral hydrate and alkali, which will result
to evolution of much chloroform or cases where dangerous or poisonous
substances are produced.
In case the
physician is to be approached about dangerous incompatibilities or
doses, always approach him armed with accepted authorities like
dispensatories, pharmacopoeias or Scovilles Art of Compounding.
In a general way
it might be stated that when the two substances likely to react are
diluted as much as possible before mixing, the chances of
incompatibility are minimized.
Another important
question in compounding is “When is the pharmacist justified in
Changing a prescription? The Answer is that a change is justified
only when such change improves the prescription without altering the
therapeutic value or action of the preparation, as in the following
cases:
- An aromatic water, when used to dissolve a salt like potassium bromide, has some of its oil separate, and a cloudly mixture results. If the salt is dissolve in the smallest quantity of water and then mixed with the aromatic water no oil will separate.
- A prescription calling for phenol mixed with an insufficient amount of water to dissolve it will form a separate layer. But if a part of the water – say 1/8 or ¼ be replaced by glycerin, a perfect solution is formed.
- A prescription calling for a certain alcoholic tincture mixed with gummy substances may precipitate the gum. If the equivalent amount of fluidextract is used, the precipitation may be avoided.
- Pills calling for ferrous sulfate, USP, or other crystalline salts are found to yield a mass that is too soft. By using the proportionate quantity of exsicated salt the mass proper consistency can be easily made.
- A prescription calling for an alkaloidal salt to be dissolved in liquid petrolatum will result in insolubility. But by using the proportionate amount of the alkaloid a good solution will be produced.
As a general
rule, it is always best to let the physician whenever a change is
made in the prescription, and always explain why the change was made.
